In the manufacture of semiconductor wafers it is common to utilize machines to handle the semiconductor wafers. Semiconductor wafers are quite valuable. Each semiconductor wafer may represent as much as eighty thousand dollars worth of product. Because the semiconductor wafers are fragile they must be handled with care so that they are not damaged during the manufacturing process. For this reason wafer handling robots are commonly used to move semiconductor wafers inside manufacturing equipment.
Wafer handling robots must be calibrated so that they are capable of moving a semiconductor wafer between a precisely located pick up point and a precisely located delivery point. The process of calibrating the operation of a wafer handling robot is referred to as “teaching” the robot. After a wafer handling robot has been calibrated or “taught” it may function properly for a period of time and then need to be recalibrated. It may be necessary to perform the teaching process several times a year for each robot. It is well known that a manually performed teaching process is time consuming and subjective when done without calibration tools. It may take an expert equipment technician several hours to successfully teach a typical wafer handling robot.
The process of calibrating a wafer handling robot and a wafer cassette may comprise (1) a process of calibrating the wafer handling robot to the wafer cassette, and (2) a process of calibrating the wafer cassette to the wafer handling robot.
For purposes of illustration, consider a portion of an exemplary prior art robot arm 100 shown in FIG. 1. Robot arm 100 comprises a base 110, a pivotally mounted first robot arm section 120, a pivotally mounted second robot arm section 130, and a pivotally mounted third robot arm section 140. The first robot arm section 120 pivots with respect to base 110. The second robot arm section pivots with respect to the first robot arm section 120. The third robot arm section 140 pivots with respect to the second robot arm section 130. The free end of the third robot arm section 140 comprises two spaced apart extensions 150 for gripping and holding a semiconductor wafer (not shown in FIG. 1).
During the semiconductor manufacturing process it is common to store the semiconductor wafers in a wafer cassette. A wafer cassette comprises a housing for holding a plurality of semiconductor wafers. An exemplary prior art wafer cassette 210 and exemplary prior art robot arm 100 are illustrated in FIG. 2.
The interior walls of wafer cassette 210 are constructed so that they form a plurality of wafer slots 220. Each wafer slot 220 is adapted to receive and hold a semiconductor wafer 230. The robot arm 100 is operated to precisely place a semiconductor wafer 230 in an empty wafer slot 220. Upon receiving an appropriate command, the robot arm 100 is capable of retrieving a designated semiconductor wafer 230 from its respective wafer slot 220.
The process of calibrating the alignment of the robot arm 100 to the wafer cassette 210 is referred to as “robot to cassette calibration” or simply “cassette calibration.” During the cassette calibration process a robot blade that is mounted on the end of robot arm 100 is employed. The robot blade is inserted between two successive semiconductor wafers 230 that are located within wafer slots 220 of the wafer cassette 210.
The technician who is performing the calibration procedure makes adjustments to the calibration of the robot arm 100 by centering the robot blade between the two semiconductor wafers 230. This requires the technician to observe the position of the robot blade when it is located between the two semiconductor wafers 230. Because the centering process is manually performed by the technician using his subjective observation, there may be “human error” inadvertently introduced during the centering process. It is very difficult for the technician to accurately determine exactly where the end of the robot blade is properly centered.
For example, consider the prior art robot blade 300 shown in FIG. 3. FIG. 3 illustrates a side view of prior art robot blade 300 of robot arm 100 while robot blade 300 is inserted between two semiconductor wafers, 230a and 230b. For purposes of clarity, the walls of the wafer cassette 210 and the wafer cassette slots 220 are not shown in FIG. 3. As may be seen in FIG. 3, the end of robot blade 300 is closer to the top surface of lower wafer 230b than to the bottom surface of upper wafer 230a. That is, the end of robot blade 300 is not centered halfway between the two semiconductor wafers, 230a and 230b. The fact that the end of robot blade 300 is not properly centered may also be seen in the end view of robot blade 300 (and semiconductor wafers, 230a and 230b) shown in FIG. 4.
The end of the robot blade 300 does not touch either the top of lower semiconductor wafer 230b or the bottom of upper semiconductor wafer 230a. For this reason, the technician may erroneously assume that the end of the robot blade 300 is properly centered when it is not, in fact, properly centered.
There is also another source of observation error. The process of viewing the location of the robot blade 300 between the two semiconductor wafers 230a and 230b can be distorted by a parallax effect caused by the reflection of an image of the robot blade 300 in the mirror surface of one of the semiconductor wafers.
FIG. 5 illustrates this problem. FIG. 5 illustrates a top perspective view of robot blade 300 located between the upper semiconductor wafer 230a and the lower semiconductor wafer 230b. The reflected image 410 of the robot blade 300 appears as a reflection in the upper surface of the lower semiconductor wafer 230b. The presence of the reflected image 410 may cause the observational judgment of the technician to be in error.
In view of the deficiencies of the prior art method, there is a need for a system and method that is capable of accurately aligning a semiconductor wafer handling robot with respect to a semiconductor wafer cassette. There is also a need in the art for a system and method that is capable of minimizing the human error involved during an alignment of a semiconductor wafer handling robot with respect to a semiconductor wafer cassette.
Before undertaking the Detailed Description of the Invention below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like.
Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior uses, as well as to future uses, of such defined words and phrases.